Linear beam tubes have been used in sophisticated communications and radar systems which require amplification of an RF or microwave electromagnetic signal. An example of a linear beam tube microwave amplifier is a conventional klystron. A conventional klystron comprises a number of cavities divided into, essentially, three sections, an input section, a buncher or amplification section, and an output section. An electron beam is sent through the klystron, and the buncher section amplifies the modulation on the electron beam and produces a highly bunched beam which contains an RF current. Various improvements in conventional klystrons have been attempted to increase bandwidth and/or efficiency. U.S. Pat. No. 3,375,397 and U.S. Pat. No. 4,284,922 disclose such klystrons.
An example of a high performance broad band klystron cavity arrangement is disclosed in continuation application Ser. No. 07/106,976, filed Oct. 1, 1987 which is a continuation of parent application Ser. No. 06/663,801, filed Oct. 23, 1984 which is hereby incorporated by reference. Also incorporated by reference is U.S. patent application Ser. No. 07/201,560 entitled "Coupled Cavity Circuit with Increased Iris Resonant Frequency." Both of the aforementioned applications are owned by the common owner [for a detailed explanation of broad band klystrons, refer to IEEE, Vol. 70, No. 11, November, 1982, pp. 1308-1310].
The invention disclosed in U.S. continuation application Ser. No. 07/106,976 is a clustered cavity klystron in which the bunching or amplification section produces a high RF power gain over a broad bandwidth.
In klystrons, the bandwidth is usually limited by the bandwidth of the output section. Prior art output sections of klystrons employing a single cavity interacting with the electron beam and a filter cavity (also called "resonator") to provide a double-tuned-circuit response have been used. In addition, klystron output circuits having more than one cavity interacting with the electron beam, which are termed in the art as extended-interaction output circuits (EIOC), have also been employed. EIOCs have the advantage that energy can be removed from the electrons over a wide band of frequencies because there is less voltage at each of the gaps of the EIOC, even though the total energy (voltage) change experienced by an electron beam can be the same as that provided by a single gap with a higher radio frequency voltage. High efficiency EIOCs are particularly necessary in use with a high gain broad bandwidth clustered cavity arrangement as disclosed in U.S. continuation application Ser. No. 07/106,976. Designers of prior art EIOCs which have been built and designed in the past, [such as that described in Mann, J. "Extended Interaction Resonator Development"]recognized that having two gaps at the output of a klystron is advantageous because the gaps act in series with the modulated electron stream traveling therethrough, thereby causing a low voltage drop across each gap, increasing bandwidth and diminishing power loss. Such prior art EIOCs have been designed by way of trial and error parameter selection followed by empirical, analytic methods. Prior art EIOCs have been limited to two cavities having a certain bandwidth and efficiency. Due to the trial and error technique of parameter selection and therefore building of such output circuits, the prior art has been unable to develop more efficient EIOCS having two, three, or more cavities.
It is therefore an object of the present invention to provide a two and three or more cavity electromagnetic output tube circuit for extracting from an electron beam amplified RF electromagnetic energy, such as that amplified within klystrons and traveling wave tubes which, has a broad bandwidth and a high RF power output.
It is a further object of the present invention to provide a unique electromagnetic output circuit which may be designed without the need for extensive trial and error parameter selection.
Further objects of the present invention will become apparent after a reading of the foregoing specification.
The aforementioned objects are achieved in an electromagnetic output circuit for outputting RF electromagnetic energy to a transmission means, the electromagnetic output circuit receiving a modulated electron beam and producing RF electromagnetic energy. The electromagnetic output circuit comprising a first cavity, the first cavity having a gap for permitting the traveling therethrough of the modulated electron beam and coupling means for permitting the traveling there-through of the electromagnetic energy. The invented electromagnetic output circuit also includes a second cavity which is coupled to the first cavity, the second cavity having a second gap for permitting the traveling therethrough of the modulated electron beam and a second coupling means for permitting the traveling therethrough of the electromagnetic energy. The distance between the first gap and the second gap is sufficient to cause a phase shift in the modulated electron beam between the first and second gaps which is substantially equal to the phase shift occurring in the electromagnetic wave between the first and second gaps; wherein the volume of the first and second cavities and the dimensions of the gaps and the first and second coupling means are proportioned such that the image impedance of the electromagnetic output circuit is approximately twice the magnitude of its output load impedance.
The above-described electromagnetic output circuit may also comprise a third cavity, the third cavity being coupled to the second cavity and having a third gap for permitting the traveling therethrough of the modulated electron beam, the third cavity also having a third coupling means for permitting the traveling therethrough of the electromagnetic energy, the distance between the second and third gaps being sufficient to cause a phase shift in the modulated electron beam between the second and third gap which is substantially equal to the phase shift occurring in the electromagnetic wave between the second and third gaps. The first, second and third cavities, the first, second and third gaps, and the first, second and third coupling means act as two microwave filter sections having first and second image impedances, respectively, wherein the second image impedance is one half the magnitude of the first image impedance and wherein the output impedance is one third the magnitude of the first image impedance.